This invention relates to impact treatment equipment, in particular to air blast equipment, wherein particulate material is conveyed in an air stream to a nozzle through which it is accelerated onto the workpiece being processed. In particular, though not exclusively, it is applicable to shot peening equipment where impact on the workpiece component of spherical particles results in a compressively stressed layer being created at the component surface, which will considerably extend the component's resistance to fatigue failure under alternating or fluctuating tensile stresses.
In order to achieve the optimum benefits and consistency from the shot peening process it is essential that the size of the spherical particles and the impact energy onto the component surface be closely controlled.
The control of the particle size is a function of the original grading of the material coupled with continual size and shape classification as it is recovered after impact for reuse.
The impact energy on the work surface is dependent upon the density of feed up to the nozzle, and the air pressure at the entry to the nozzle. As an alternative to the latter, the velocity of the particle stream in the hose to the nozzle can be used since either parameter is related to the velocity of the stream leaving the nozzle.
Normal practice is to control the feed density by means of an adjustable feed valve between the media storage hopper, and the hose leading to the nozzle; the nozzle discharge velocity is controlled by means of a pressure regulator in the compressed air supply line. The required setting of the feed valve and pressure regulator are established so as to achieve the required impact of the particles on the component to obtain the optimum peening effect, and are preset whenever the process is repeated.
While this preset condition can be checked at the start and finish of the processing of each component by means of the curvature produced on Almen Test strips in order to verify that the required impact intensity is produced, deviations may occur during the processing which may not be detected. For example, moisture in the compressed air supply can result in coagulation of the peening media and cause irregular flow of media through the feed valve. A similar effect can result if oversize debris accidentally contaminates the media. Furthermore the compressed air supply pressure may fall momentarily below the regulated setting.
These and other conditions may result in undetected changes in the peening intensity during a processing cycle, and therefore in loss of consistency of performance. An object of the present invention is to ensure consistency of performance.